While Android 4.2 is now making its way into AOSP (Android Open Source Project), support for one device in that code is not, and won't be for some time: the Nexus 4. Android maintainer JBQ (an awesome guy we quote a lot on days like this) has confirmed that the "Nexus 4 is not supported in AOSP at the moment: no source files, no binaries."

JBQ is not able to comment on why this is the case, or when we might expect the Nexus 4 to join Google's big happy AOSP Nexus device family, though obviously if Google could merge it into AOSP today, they would.